A mounted canvas or sheet, the object of which is to show an image on the image side thereof, such as a poster, a photograph, a thin (roll-up) screen, a projection screen and the like, should preferably remain flat during use.
One problem when mounting such a canvas or sheet is the fact that the latter cannot easily be mounted so that it is truly flat. Every canvas or sheet has a tendency to curl along the edges when it is being mounted. This is due to the fact that the tension along the edges is lower than in the centre, resulting in the edges between the corners being slack. Thus, the edges may start to curl or flap. Wrinkling also occurs easily and a conventionally mounted canvas or sheet is sensitive to external factors, such as pressure, temperature and humidity, as a result of which the canvas or sheet may deform. Partly due to this, the canvas or sheet is not perfectly flat, despite having been mounted.